Conventionally, for the purpose of reducing a rolling resistance of a tire which has a strong connection to a good mileage of a vehicle, there has been known a pneumatic tire in which a tread rubber is highly compounded with a silica. This is structured by reducing a compounding amount of a carbon black causing a hysteresis loss and compounding the silica alternatively. Accordingly, it is possible to reduce the rolling resistance and achieve the good mileage, and it is possible to increase a braking force on a wet road so as to improve a wet performance.
However, the tread rubber mentioned above has a higher electric resistance in comparison with the tread rubber in which only the carbon black is compounded, and has a problem that if a static electricity and an electricity generated by an internal friction at a time when a rubber is deformed or the like are accumulated in a vehicle body, there is generated a problem such as an earth to the human body, a radio noise or the like. Accordingly, there has been developed a pneumatic tire structured such that a conductive layer in which the carbon black or the like is compounded is provided in a non-conductive tread rubber in which the silica or the like is compounded, thereby discharging the electricity generated in the vehicle body to the road.
For example, the following Patent Document 1 describes a pneumatic tire in which a non-conductive tread rubber is provided with a conductive layer extending to an outer side in a tire diametrical direction from a bottom surface thereof so as to reach a tread surface. However, in the tire mentioned above, there is a problem that a forming position of the conductive layer is affected by a pattern design, and there is a case where a conductivity is insufficient in the other pattern designs than the rib pattern continuously extending in the tire circumferential direction. Further, in the case where the conductive layers mentioned above are formed at plural numbers, there is a problem that a manufacturing step becomes complicated.
On the other hand, there can be considered to apply a liquid material such as a rubber paste or the like having a conductive rubber compounded to a region reaching a bottom surface from both sides in a width direction of a tread surface via a side surface, thereby forming a conductive layer and applying a conductive performance. However, in the tire mentioned above, there is a case where the conductive performance cannot be sufficiently achieved in some wear state of the tread rubber such as a case where only a shoulder is worn in an early stage.
The following Patent Document 2 describes a pneumatic tire in which a conductive layer extends in a lottery shape or a net shape from a tread surface to a bottom surface. The tread rubber is formed by partly or wholly covering a surface of a non-conductive rubber ribbon with a conductive layer, and winding and laminating it along a tire circumferential direction. However, in the tire mentioned above, since the conductive rubber layer is continuously exposed onto the tread surface in correspondence to the wear state as well as a rate of the conductive rubber is increased more than necessary, there is a case where the good mileage and the wet performance corresponding to an original purpose cannot be achieved.
The following Patent Document 3 describes a pneumatic tire in which a non-conductive tread rubber is provided with a conductive layer which continuously and spirally extends along a tire circumferential direction, and in which one end is exposed on a tread surface and the other end comes into contact with a sidewall rubber. The tread rubber is formed by winding a non-conductive rubber ribbon along a tire circumferential direction, forming a rubber layer of a predetermined cross sectional shape having a step portion, thereafter winding the conductive rubber ribbon on the rubber layer, and further winding the non-conductive rubber ribbon thereon. However, in the tire mentioned above, there is a case where the conductive rubber exposed on the tread surface is extremely reduced in some wear state of the tread rubber, and there is a risk that the conductive performance cannot be sufficiently secured. Further, there is a tendency that a manufacturing step is complicated and a productivity is lowered.
Patent Document 1: DESCRIPTION of Japanese Patent No. 2944908
Patent Document 2: Japanese Unexamined Patent Publication No. H11-227415
Patent Document 3: Japanese Unexamined Patent Publication No. 2004-136808